return 400 when validation fails

db/index.ts

@@ -61,7 +61,12 @@ export async function createBooking({
   const bookedDays = await getBookedDays()
 
   if (booking.days.some((day: string) => bookedDays.includes(day))) {
-    throw new mongoose.Error.ValidationError(booking)
+    const error = new mongoose.Error.ValidationError(booking)
+    error.addError(
+      'endDate',
+      'At least on one day the van is not avaliable. Choose another startDate or endDate.'
+    )
+    throw error
   }
 
   let booker = await Booker.findOne({ email }).exec()

pages/api/booking/index.tsx

@@ -19,9 +19,8 @@ export default async function userHandler(
         res.status(200).json(booking)
       } catch (e) {
         console.error(e)
-
-        if (e instanceof Error.ValidatorError) {
-          res.status(400).end(e.message)
+        if (e instanceof Error.ValidationError) {
+          res.status(400).json({ message: e.message, errors: e.errors })
           return
         }
         res.status(500).end(`Internal Server Error...Guru is meditating...`)